The present invention relates to a robot such as articulated manipulators and more particularly to a robot having an arm which makes use of a parallel linkage.
There have been well known robots such as manipulators utilizing a four-bar linkage (e.g., parallel linkage) as an articulation mechanism. FIG. 1 is a side elevation showing a structure of a known robot of this type. The robot shown in FIG. 1 mainly comprises a bar-like arm member A, a first link 1, a second link 2, a third link 3 and a base 6.
Attached to the distal end of the arm member A is an end effector (not shown) such as a hand for gripping a workpiece or a welder for executing welding work on a workpiece. The part around the distal end of the arm member A is able to pivot about the central axis of the arm member A and flex relative to a longitudinal direction of the arm member A.
At the intermediate section of the arm member A somewhat closer to its proximal end when viewed in the longitudinal direction of the arm member A, one end of the first link 1 is pivotally connected by a pivotal shaft which extends in a direction perpendicular to the longitudinal direction of the arm member A. The other end of the first link 1, on the other hand, is pivotally supported on a swivel base 61 by a pivotal shaft extending in substantially the same direction as the above pivotal shaft.
The base 6 is composed of the swivel base 61 and a mount 62. The mount 62 houses a motor, reduction gears and others (not shown) which cause the swivel base 61 to turn in a horizontal direction.
One end of the second link 2 is pivotally connected to the proximal end of the arm member A by a pivotal shaft extending in substantially the same direction as the above pivotal shafts. The link length (i.e., the distance between two pivotal shafts) of the second link 2 is substantially the same as that of the first link 1, and the other end of the second link 2 is pivotally connected to one end of the third link 3 by a pivotal shaft which extends in substantially the same direction as the above pivotal shafts.
The other end of the third link 3 is pivotally supported on the swivel base 61 so as to be coaxial with the other end of the first link 1. The link length of the third link 3 is substantially the same as the distance between the position at which the first link 1 is pivotally connected to the arm member A and the position at which the second link 2 is pivotally connected to the arm member A. Thus, the arm member A, the first link 1, the second link 2 and the third link 3 constitute a parallel linkage.
In the swivel base 61, a motor for causing pivoting of the first link 1 and a motor for causing pivoting of the third link 3 are opposed to each other (not shown). The rotary shaft of one of the motors is coupled to the pivotal shaft of the first link 1 such that the rotation of the motor is transmitted to the first link 1, whereas the rotary shaft of the other motor is coupled to the pivotal shaft of the third link 3 such that the rotation of the motor is transmitted to the third link 3.
The lower part of the second link 2 extends in a direction opposite to the first link 1 and the distal end of the lower part is provided with a balancer weight (weight) W. The balancer weight W is provided in order that it balances the mass of the arm member A, the end effector, a workpiece to be conveyed etc. in the area around the pivotal shaft for supporting the third link 3 on the swivel base 61, so that the load imposed on the motor for causing pivoting of the third link 3 can be reduced.
A spring unit 7 housing a spring is disposed in parallel with the first link 1. One end of the spring unit 7 is pivotally connected to the pivotal shaft for connecting the arm member A and the first link 1, while the other end being pivotally connected to the upper end part of the swivel base 61. With this arrangement, when the first link 1 inclines with the arm member A moving, the spring unit 7 energizes the arm member A in a direction opposite to the moving direction, thereby reducing the load imposed on the motor for causing pivoting of the first link 1.
The conventional robot described above, however, presents the problem that, great load torque is imposed on the motor for causing pivoting of the third link 3 for the following reason, particularly where mass capacity is 100 kg or more.
FIG. 2 is a diagrammatic side elevation illustrating a dynamically balanced condition of a conventional robot. As shown in FIG. 2, where a force F is downwardly exerted on the distal end of the arm member A and the third link 3 is inclined from a horizontal position in a counter-clockwise direction in the drawing through an angle [h]xcex8, the arm member A is also inclined from a horizontal position in the same direction through the angle [h]xcex8 so that the magnitude of the component of the force F working in a direction perpendicular to the arm member A is Fcos[h]xcex8. Therefore, the force F1 expressed by Formula (1) is imposed on the proximal end of the arm member A.
F1=(L/L3)F cos [h]xcex8(1)
In this equation, L designates the length between the position where the arm member A is pivotally connected to the first link 1 and the distal end of the arm member A; and L3 designates the length between the position where the arm member A is pivotally connected to the first link 1 and the position where the arm member A is pivotally connected to the second link 2.
Since the force F1 is imposed, by way of the second link 2, on the position where the third link 3 is pivotally connected to the second link 2 and the link length of the third link 3 is L3, load torque t having a magnitude represented by F1xc2x7L3 (=Lxc2x7Fcos[h]xcex8) is imposed on the motor M for causing pivoting of the third link 3.
Therefore, where the robot carries a massive workpiece, that is, where the robot has a mass capacity of 100 kg or more, the load on the motor M becomes extremely great which gives rise to a need for a large-sized motor.
In addition, in the above case, since great torque is exerted on the pivotal axis which pivotally supports the third link 3 on the base 6, great stresses are generated in the third link 3, its pivotal shaft and peripheral members. In order to ensure rigidity high enough to withstand the stresses, the sizes of these members are inevitably increased.
In the conventional robot, not only the capacity of the motor M needs to be increased but also the reduction gears coupled to the rotary shaft of the motor M must have a high reduction ratio. As a result, the conventional robot cannot transport a workpiece at high speed.
Another problem presented by the conventional robot is such that while the load on the motor M can be reduced by providing a balancer weight W which can be balanced against the mass of the arm member A, the end effector, the workpiece to be conveyed and others, the balancer weight W should be large in size where the mass capacity is 100 kg or more so that the mass of the robot 1 itself increases, giving rise to a need for a large-sized motor for causing slue of the swivel base 61 in a horizontal direction.
A robot using the so-called hybrid linkage suffers from the following problems.
FIG. 3 schematically, diagrammatically illustrates a side elevation of a robot using the so-called hybrid linkage. The robot using the hybrid linkage has a parallel linkage composed of four links L11 to L14. Of these links L11 to L14, the laterally extending upper link L11 is provided with an extension part which extends upwardly from the intermediate section of the link L11 when viewed in a longitudinal direction. The extension part is bent midway at substantially right angles, and a motor and reduction gears (not shown) are attached to the distal end of the extension part. The rotary shaft of the motor is coupled to the intermediate section of the arm member A through the reduction gears. Thus, the hybrid linkage is constituted by the links L11 to L14 and the arm member A.
Of the links L11 to L14, the vertically extending link L12 positioned on the side of the distal end of the arm member A and the laterally extending lower link L14 are coaxially pivotally supported on a base 6, and the link L12 is caused to pivot by a motor and reduction gears (not shown) housed in the base 6.
When carrying a massive workpiece (e.g., mass capacity is 100 kg or more) with the above-described robot employing the hybrid linkage, an extremely great load is imposed on the motor for causing pivoting of the arm member A. In order to reduce the load on the motor for the arm member A, the proximal end of the arm member A needs to be extended for attachment of a balancer weight W as shown in FIG. 3, which, in turn, leads to an increase in the load on the motor for causing pivoting of the link L12. Therefore, in the case of the robot employing the hybrid linkage, the loads imposed on the motors extremely increase, for instance, where mass capacity is 100 kg or more so that there arises a need for large-sized motors.
The problems encountered by a robot using the so-called serial linkage will be discussed below.
FIG. 4 schematically diagrammatically shows a side elevation of a robot utilizing the serial linkage. The robot using the serial linkage is designed such that, one end of a link L21 is pivotally connected to the intermediate section of an arm member A while the other end of the link L21 being pivotally supported on a base 6. The arm member A and the link L21 thus constitute the serial linkage. Attached to the distal end of the link L21 are a motor and reduction gears (not shown) which cause pivoting of the arm member A. The base 6 also houses a motor and reduction gears (not shown) which cause pivoting of the link L21.
In the robot having such a serial linkage, where a massive workpiece is transported (e.g., mass capacity is 100 kg or more), an extremely great load is imposed on the motor for causing pivoting of the arm member A, and it becomes necessary to extend the proximal end of the arm member A for attachment of a balancer weight W similarly to the case of the hybrid linkage robot in order to reduce the load on the motor for the arm member A. In this case, not only the weight of the workpiece to be conveyed but also the weight of the balancer weight W are imposed as a load on the motor for causing pivoting of the link L21. Therefore, where mass capacity is 100 kg or more in the case of the robot employing the serial linkage, the loads imposed on the motors extremely increase, giving rise to a need for use of large-sized motors.
The invention is directed to overcoming the foregoing drawbacks and a primary object of the invention is therefore to provide a robot in which loads on motors can be reduced without use of a large-sized balancer weight, and motors and various parts smaller in size than those of the conventional robot can be used, and high-speed conveyance of workpieces is enabled.
The above object can be achieved by a robot according to the invention comprising: a base; a bar-shaped arm member having a distal end and a proximal end; a first link one end of which is pivotally connected to an intermediate section of the arm member while the other end being pivotally supported on the base; a second link one end of which is pivotally connected to the proximal end of the arm member; a third link one end of which is pivotally connected to the other end of the second link while the other end being pivotally supported on the base so as to be coaxial with the other end of the first link; a fourth link one end of which is pivotally connected to at least either one of the other end of the second link and the one end of the third link; a fifth link one end of which is pivotally connected to the other end of the fourth link while the other end being pivotally supported on the base, wherein the link length of the fifth link being shorter than that of the third link; and an actuator for causing the fifth link to pivot on the other end of the fifth link, wherein the arm member, the first link, the second link and the third link constitute a four bar linkage.
According to the invention, the load torque exerted on the pivotal shaft for the fifth link can be reduced to a considerable extent, compared to the load torque imposed on the pivotal shaft for the third link of the conventional robot. Therefore, actuators and various parts having smaller sizes than those of the prior art can be employed and high-speed conveyance of workpieces is enabled.
In the invention, it is preferable to pivotally support the other end of the fifth link on the base at a position that is a distance away from the position where the first link and the third link are pivotally supported on the base, the distance being shorter than the link length of the third link. This ensures a sufficient movement range for the robot.
In the invention, the link length of the fifth link is preferably 0.3 to 0.8 times the link length of the third link. With this arrangement, the load torque imposed on the actuator can be prevented from excessively increasing and the movement range for the arm member can be prevented from becoming excessively small.
In the invention, the link length of the fourth link is preferably 0.9 to 1.1 times the link length of the fifth link. With this arrangement, the movement range for the arm member can be prevented from becoming excessively small, the size of the robot itself can be prevented from increasing, and practically sufficient rigidity can be ensured for the area where the fourth link and the fifth link are coupled to each other.
In the invention, it is preferable to set the mass capacity of the robot to 100 kg or more. By virtue of this, a reduction in the production cost for the robot having a mass capacity of 100 kg or more can be expected. In addition, the balancer weight can be more miniaturized and an increase in the size of the robot can be restrained.